1. Field of the Invention
The present invention relates generally to rotating drilling head systems in which an elastomer seals around and grips a rotating drill pipe during drilling operations.
2. Description of the Related Art
Oil and gas wells are typically drilled by use of a rotating drill pipe with a drill bit at the lower end. Drilling fluids are pumped down the drill pipe and out the drill bit. The drilling fluid returns to the surface, along with cuttings, through the annulus around the drill pipe. In many cases, the pressure at the upper end of the drill pipe annulus is atmospheric. The weight of the drilling fluid is controlled to provide a hydrostatic pressure at the earth formations that is greater than the formation pressure to prevent blowouts.
In some cases, however, it is advantageous to isolate the pressure at the upper end of the drilling fluid column from atmospheric pressure. For example, in highly deviated well, a lightweight drilling fluid may be used that is not heavy enough to prevent upward flow in the well due to formation pressure. A drilling head at the upper end of the well controls the pressure. Drilling head systems use an elastomeric element to seal the drilling head against the rotating drill pipe during drilling operations. In some rotating drilling head systems, the seal is formed by the natural resiliency of the elastomeric element against the drill pipe while others use hydraulic pressure to deform the seal element. In U.S. Pat. No. 6,016,880, hydraulic pressure to energize an elastomeric gripper element that is located above an elastomeric primary seal. The gripper grips the drill pipe to cause the gripper and primary seal to rotate with the drill pipe. The gripper also serves as a secondary seal in the event of leakage of the primary seal.
The primary seal of the '880 patent and in other prior art normally comprises an elastomeric seal with a tapered exterior that is exposed to drilling fluid pressure. The drill string has enlarged tool joint sections at the end of each drill pipe that must pass through the interior of the seal. The drilling fluid pressure and movement of the drill pipe through the seal causes extrusion of the seal, which limits the life of the seal.